Membrane loop process for separating carbon dioxide for use in gaseous form from flue gas

ABSTRACT

The invention is a process involving membrane-based gas separation for separating and recovering carbon dioxide emissions from combustion processes in partially concentrated form, and then transporting the carbon dioxide and using or storing it in a confined manner without concentrating it to high purity. The process of the invention involves building up the concentration of carbon dioxide in a gas flow loop between the combustion step and a membrane separation step. A portion of the carbon dioxide-enriched gas can then be withdrawn from this loop and transported, without the need to liquefy the gas or otherwise create a high-purity stream, to a destination where it is used or confined, preferably in an environmentally benign manner.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/123,364, filed on Apr. 8, 2011, which is a national stage applicationof, and claims the benefit of, PCT Application No. PCT/US2010/002479,filed on Sep. 13, 2010 the disclosures of which are hereby incorporatedherein by reference in their entireties.

STATEMENT OF GOVERNMENTAL SUPPORT

The invention was made in part with U.S. Government support under SBIRAward No. DE-NT-000-5312, awarded by the U.S. Department of Energy. TheU.S. Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to membrane-based gas separation processes, andspecifically to loop processes for recycling carbon dioxide in gaseousform from flue gas, where the recovered carbon dioxide can betransported for use or confinement in partially concentrated form.

BACKGROUND OF THE INVENTION

Many combustion processes produce flue gases contaminated with carbondioxide that contribute to global warming and environmental damage. Suchgas streams are dilute, low-pressure, and difficult to treat; therefore,this gas cannot be economically transported to places where the carbondioxide could be used.

To make transportation more feasible, the carbon dioxide can beseparated and enriched to produce 90-100% pure carbon dioxide. Whilethis concentrated carbon dioxide can be easily transported, theseparation processes required to concentrate the carbon dioxide remainexpensive. As such, there remains a need for better treatment options.

Gas separation by means of membranes is a well-established technology.In an industrial setting, a total pressure difference is usually appliedbetween the feed and permeate sides, typically by compressing the feedstream or maintaining the permeate side of the membrane under partialvacuum.

It is known in the literature that a driving force for transmembranepermeation may be supplied by passing a sweep gas across the permeateside of the membranes, thereby lowering the partial pressure of adesired permeant on that side to a level below its partial pressure onthe feed side. In this case, the total pressure on both sides of themembrane may be the same, the total pressure on the permeate side may behigher than on the feed side, or there may be additional driving forceprovided by keeping the total feed pressure higher than the totalpermeate pressure.

Using a sweep gas has most commonly been proposed in connection with airseparation to make nitrogen- or oxygen-enriched air, or withdehydration. Examples of patents that teach the use of a sweep gas onthe permeate side to facilitate air separation include U.S. Pat. Nos.5,240,471; 5,500,036; and 6,478,852. Examples of patents that teach theuse of a sweep gas in a dehydration process include U.S. Pat. Nos.4,931,070 and 5,641,337.

Configuring the flow path within the membrane module so that the feedgas and sweep stream flow, as far as possible, countercurrent to eachother is also known, and taught, for example in U.S. Pat. Nos. 5,681,433and 5,843,209.

The use of a process including a membrane separation step operated insweep mode for treating flue gas to remove carbon dioxide is taught inco-owned and copending patent application Ser. No. 12/734,941, filedJun. 2, 2010.

Despite the innovations described above, the problem of capturing andsequestering carbon dioxide, so as to prevent its release to theatmosphere, or at least to delay it for many years, remains verydifficult to solve in an energy- and cost-efficient manner. Solutionsthat have been proposed often rely on recovering the carbon dioxide inessentially pure form, such as by subjecting a stream containing thecarbon dioxide to cryogenic distillation or amine sorption, followed byliquefaction. Although these solutions are potentially useful for someapplications, there remains a need for a relatively simple, low-energysolution for treating carbon dioxide streams that avoids the requirementto create a high-purity stream.

SUMMARY OF THE INVENTION

The invention is a process involving membrane-based gas separation forseparating and recovering carbon dioxide emissions from combustionprocesses in partially concentrated form, and then transporting thecarbon dioxide and using or storing it in a confined manner withoutconcentrating it to high purity.

Combustion exhaust streams or off-gases are typically referred to asflue gas, and arise in large quantities from ovens, furnaces, boilers,and heaters in all sectors of industry. In particular, power plantsgenerate enormous amounts of flue gas. A modestly sized 100 megawattcoal-based power plant may produce over 300 MMscfd of flue gas.

The major components of combustion exhaust gases are normally nitrogen,carbon dioxide, and water vapor. Other components that may be present,typically only in small amounts, include oxygen, hydrogen, SO_(x),NO_(x), and unburnt hydrocarbons. The carbon dioxide concentration inthe flue gas is generally up to about 20 vol %.

In addition to gaseous components, combustion flue gas containssuspended particulate matter in the form of fly ash and soot. Thismaterial is usually removed by several stages of filtration before thegas is sent to the stack. It is assumed herein that the flue gas hasalready been treated in this way, if desired, prior to carrying out theprocesses of the invention.

The process of the invention involves treating the exhaust or flue gasto remove carbon dioxide. In preferred embodiments, the carbon dioxidelevel of the exhaust gas is reduced to as low as 5 vol % or less, andmost preferably to 3 vol % or less, or even 2 vol % or less. Dischargeof such a stream to the environment is much less damaging than dischargeof the untreated exhaust.

The process of the invention further involves building up theconcentration of carbon dioxide in a gas flow loop between thecombustion step and the membrane separation step. A portion of thecarbon dioxide-enriched gas can then be withdrawn from this loop andtransported, without the need to liquefy the gas or otherwise create ahigh-purity stream, to a destination where it is used or confined,preferably in an environmentally benign manner. One preferred option isto direct the gas to an operation or facility that converts the carbondioxide to oxygen by photosynthesis, such as an algae farm. A secondpreferred option is to use the gas for enhanced oil recovery. A thirdpreferred option is to use the gas to enhance the recovery of coalbedmethane. A fourth preferred option is to use the gas to precipitatecalcium and magnesium in sea water, to immobilize the carbon dioxide andpotentially produce cement and aggregate. A fifth preferred option is toinject the gas into a sub-surface aquifer or salt brine layers tosequester the carbon dioxide, thereby reducing its potentialcontribution to global warming.

The combustion process from which the exhaust is drawn may be of anytype. The fuel may be a fossil fuel, such as coal, oil, or natural gas,or may be from any other source, such as landfill gas, syngas, biomass,or other combustible waste. The fuel may be combusted by mixing withair, oxygen-enriched air, or pure oxygen.

The combustion process produces an exhaust gas, off-gas, or flue gas,which is sent for treatment in a membrane separation unit. The unitcontains membranes selectively permeable to carbon dioxide overnitrogen, and to carbon dioxide over oxygen. It is preferred that themembrane provide a carbon dioxide permeance of at least about 300 gpu,more preferably at least about 500 gpu, and most preferably at leastabout 1,000 gpu under the operating conditions of the process. A carbondioxide/nitrogen selectivity of at least about 10, or more preferably20, under the operating conditions of the process is also desirable.

The off-gas flows across the feed side of the membranes, and a sweep gasof air, oxygen-enriched air, or oxygen flows across the permeate side,to provide at least part of the driving force for transmembranepermeation. It is preferred that the feed gas flow direction across themembrane on the feed side and the sweep gas flow direction across themembrane on the permeate side are substantially countercurrent to eachother. In the alternative, the relative flow directions may besubstantially cross-current or less preferred, cocurrent.

The process may be augmented by operating the membrane unit with highertotal pressure on the feed side than on the permeate side, therebyincreasing the transmembrane driving force for permeation. Slightcompression of the feed stream to a pressure from between about 1.5 barup to about 5 bar, such as 2 bar, is preferred.

The sweep stream picks up the preferentially permeating carbon dioxide.The combined sweep/permeate stream is then withdrawn from the membraneunit and is returned to the combustor to form at least part of the air,oxygen-enriched air, or oxygen feed to the combustion step.

The membrane separation step may be carried out using one or moreindividual membrane modules. Any modules capable of operating underpermeate sweep conditions may be used. Preferably, the modules take theform of hollow-fiber modules, plate-and-frame modules, or spiral-woundmodules. All three module types are known, and their configuration andoperation in sweep, including counterflow sweep modes, is described inthe literature.

The process may use one membrane module, but in most cases, theseparation will use multiple membrane modules arranged in series and/orparallel flow arrangements, as is well-known in the art. Any number ofmembrane modules may be used.

The residue stream from the membrane separation step forms the treatedexhaust gas, preferably containing less than about 5 vol % carbondioxide, as mentioned above. This stream is typically, although notnecessarily, discharged to the environment. The substantial reduction ofthe carbon dioxide content in the raw exhaust greatly reduces theenvironmental impact of discharging the stream.

Return of the carbon dioxide-enriched permeate stream from the membraneseparation step to the combustion step forms a gas flow loop between thecombustor and the membrane separation unit, with flue gas from thecombustor flowing to the membrane separation step, and permeate gas fromthe membrane separation step flowing back to the combustion step.

The carbon dioxide concentration in the loop builds to a considerablyhigher level than would be the concentration in the flue gas from aconventional combustion step without the membrane separation step.Typically, the carbon dioxide concentration in the loop will be enrichedseveral fold, such as three, four, five, or more times, compared withthe carbon dioxide concentration that would be found in exhaust gas froma combustion step operated without the loop configuration. For example,a natural gas-fired, combined cycle power plant typically produces aflue gas with about 4-5 vol % carbon dioxide. Using the loop process ofthe invention, the carbon dioxide concentration of the flue gas maytypically be built up to at least about 10, 20, 25, or 30 vol % or more.Similarly, the exhaust gas from an oil- or coal-fired power plantgenerally contains about 12-15 vol % carbon dioxide, and can typicallybe built up to at least about 20, 30, 40, 50 or more vol % carbondioxide in the membrane unit/combustor loop.

Carbon dioxide is withdrawn from the loop as a carbon dioxide-enrichedproduct, draw, or bleed stream. The process can be configured to tailorthe concentration of this stream so that the stream can be used ingaseous form, without concentration to high purity and withoutliquefaction. Because the loop incorporates the combustion step, andcirculates oxygen and nitrogen as well as carbon dioxide, the carbondioxide concentration in the loop is usually below 60 vol %, and no moretypically than about 50 vol %. In other words, the process of theinvention produces a partially concentrated bleed stream.

The process is thus distinguished from processes that require the carbondioxide to be further concentrated to a purity of 95 vol % or above.Such processes usually involve either liquefaction of the carbon dioxideby low-temperature distillation, or absorption of the carbon dioxideinto a chemical or physical sorbent, followed by regeneration of thesorbent to yield a high-purity carbon dioxide product. The process ofthe invention does not require these steps, so is often much lower inenergy consumption than such processes.

If the draw stream taken from the loop requires further concentration,this may be carried out using additional membrane separation steps,operated in any convenient manner.

The carbon dioxide-enriched stream withdrawn from the loop is usedwithout liquefaction, preferably in a process that breaks down thecarbon dioxide or that stores it in a confined way that reduces oreliminates its emission to the atmosphere. Representative, butnon-limiting uses include algae fainting, enhanced oil recovery,enhanced coalbed and coal mine methane recovery, carbon dioxidemineralization for aqueous precipitation, and direct injection intounderground aquifers, oceans, or brine lakes.

A number of processes exist (e.g., algae farms, enhanced oil or coalbedmethane recovery operations) that can utilize carbon dioxide recoveredfrom combustion flue gas. However, because such flue gas isdilute—typically containing 5 to 15 vol % carbon dioxide—it is noteconomically feasible to transport the gas to a location where it caneffectively be put to use. The conduit diameter required would be verylarge, and blowers needed to push the gas through the conduit wouldrequire a huge amount of energy.

One alternative is to separate the carbon dioxide from the nitrogen andother components of the flue gas, producing 95% pure carbon dioxide,which can he pressurized and liquefied for easy transportation.Unfortunately, the cost of separation and pressurization is prohibitivefor many end uses.

According to a preferred process of the invention, a gas stream isproduced that contains approximately 30 to 50 vol % carbon dioxide atvery low cost. This gas stream can then be used on-site or may betransported to a location where it can be put to effective use. If thegas needs to be transported, transportation of the concentrated gas isfar more economical than transporting conventional, untreated flue gas.Typically, the volume of gas that must be pipelined is reduced five-foldor more.

In many cases, this gas can be used in the final process without furtherconcentration of the carbon dioxide; for example, in most algae farms,coalbed or coal mine methane operations, processes such as the Caleraprocess that mineralizes carbon dioxide by conversion into cementaggregate and carbonate solids by aqueous precipitation of calcium andmagnesium in seawater, and some enhanced oil recovery applications. Forsome processes, further concentration of the gas may be required, butthis is easier starting with 30 to 50 vol % concentration carbon dioxidethan with dilute, untreated flue gas.

A basic embodiment of a process of the invention includes the followingsteps:

-   -   (a) performing a combustion step by combusting a mixture of a        fuel and air, oxygen-enriched air, or oxygen, thereby creating        an exhaust stream comprising carbon dioxide and nitrogen;    -   (b) providing a membrane having a feed side and a permeate side,        and being selectively permeable to carbon dioxide over nitrogen        and to carbon dioxide over oxygen;    -   (c) performing a membrane separation step, comprising,        -   (i) passing at least a portion of the exhaust stream across            the feed side,        -   (ii) passing air, oxygen-enriched air, or oxygen as a sweep            stream across the permeate side,        -   (iii) withdrawing from the feed side a carbon            dioxide-depleted vent stream, and        -   (iv) withdrawing from the permeate side a permeate stream            comprising oxygen and carbon dioxide;    -   (d) passing at least a portion of the permeate stream to        step (a) as at least part of the air, oxygen-enriched air, or        oxygen used in step (a), thereby forming a gas-flow loop between        the combustion step and the membrane separation step;    -   (e) withdrawing from the gas flow loop a carbon        dioxide-enriched, partially concentrated gas product stream; and    -   (f) using, storing, or otherwise disposing of the partially        concentrated gas product stream in a confined manner.

Typically, the partially concentrated gas product stream is transportedto a confining operation, where it is used, stored, or otherwisedisposed of in a confined manner.

In one embodiment of the invention, the partially concentrated gasproduct stream is transported to an operation or facility that breaksdown the carbon dioxide by photosynthesis, which is typically an algaefarm. In a second embodiment of the invention, the partiallyconcentrated gas product stream is used for enhanced oil recovery. In athird embodiment of the invention, the partially concentrated gasproduct stream is used for enhanced coalbed or coal mine methanerecovery. In a fourth embodiment of the invention, the partiallyconcentrated gas product stream is used in the production of carbonates.In a fifth embodiment of the invention, the partially concentrated gasproduct stream is injected into subsurface water.

A primary objective of the invention is to control carbon dioxideemissions to the atmosphere.

A secondary objective of the invention is to use membrane separation inan energy-efficient manner to provide a residue stream of carbon dioxidecontent below 5 vol % that can be vented to the atmosphere.

Yet another objective of the invention is to create a gas streamenriched in carbon dioxide content that can either be used on-site ortransported—for example, by pipeline—in partially concentrated form, andto use this stream in an environmentally benign manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a flow scheme for a basic embodiment ofthe invention as it relates to a typical combustion process.

FIG. 2 is a schematic drawing of a flow scheme for an embodiment of theinvention, which is a variant of the flow scheme shown in FIG. 1, inwhich the exhaust stream being routed to the sweep-based membraneseparation step is passed through a compressor prior to being sent tothe membrane separation step.

FIG. 3 is a schematic drawing of a flow scheme for a combustion processthat does not include a sweep-based membrane separation step (not inaccordance with the invention).

DETAILED DESCRIPTION OF THE INVENTION

Gas percentages given herein are by volume unless stated otherwise.

Pressures as given herein are in bar absolute unless stated otherwise.

The terms confining manner and confining operation refer to processesthat use, store, or otherwise dispose of the carbon dioxide in a mannersuch as to essentially eliminate, reduce, or delay the emission of therecovered carbon dioxide to the atmosphere.

The terms exhaust gas, off-gas, flue gas, and emissions stream are usedinterchangeably herein.

With respect to streams containing carbon dioxide, the term high puritymeans containing at least about 95 vol % carbon dioxide.

With respect to streams containing carbon dioxide, the term partiallyconcentrated means containing 60 vol % carbon dioxide or less.

The invention is a process for separating and recovering carbon dioxidefrom combustion processes, and routing the recovered carbon dioxide to acontainment destination, such as an algae farm, or a containment processor step, such as an enhanced oil recovery process. In general, by theterms containment destination, containment use, and containment process,we mean any destination, use, or process that uses a carbon dioxidestream in partially concentrated form, that does not require or producea high purity carbon dioxide stream in gas or liquid form, and thatcontains or converts the carbon dioxide in such a way as to essentiallyeliminate, or at least delay for many years, the emission of therecovered carbon dioxide to the atmosphere.

A simple flow scheme for a preferred embodiment of the invention isshown in FIG. 1. From FIG. 1, it may be seen that the process of theinvention incorporates a combustion step, followed by a sweep-basedmembrane separation step. A portion of the exhaust stream from thecombustion process is routed to the sweep-based membrane separationstep, and the permeate portion of gas from the membrane separation stepis routed back to the combustion step, thereby forming a gas flow loop,indicated by number 119 in the figure. A bleed, draw, or product streamis withdrawn from the gas flow loop and routed to a process that breaksdown or stores the carbon dioxide in a confined manner, such as an algaefarm or enhanced oil recovery process.

Referring to FIG. 1, fuel stream 102 and air, oxygen-enriched air, oroxygen stream 104 are introduced into combustion step or zone 112.Stream 104 is made up of sweep stream 103 (discussed below) and,optionally, additional air or oxygen supply stream 115.

The combustion step may be carried out in any way limited only in thatit results in an off-gas, exhaust gas, or flue, gas containing carbondioxide. Such combustion processes occur throughout industrializedsociety. Representative processes include those in which the combustionstep is used to provide heat for an oven or furnace. such as a blastfurnace or rotary kiln, for example, a lime or cement kiln. Otherimportant processes are those in which the combustion step is used togenerate steam to operate a turbine or other equipment to performmechanical work or generate electric power. In yet other processes, thecombustion gases themselves are used as a source of power to drive aturbine or the like, and may be treated before or after they have beenused in the turbine. Further examples of combustion processes are thoseused to supply heat for refinery operations, such as certain types ofcracking or reforming.

The fuel for the combustion step may be any fuel that can he combustedwith oxygen, including, but not limited to, coal, coke, wood, biomass,solid wastes, oils, and other natural and synthetic liquid fuels of allgrades and types, and hydrocarbon-containing gas of any type, such asnatural gas, landfill gas, coal mine gas, or the like.

The oxygen with which the fuel is combusted may be supplied in the formof high purity oxygen, oxygen-enriched air, normal air, or any othersuitable oxygen-containing mixture.

Combustion exhaust stream 105—preferably containing at least 15 vol %;more preferably, at least 20 vol %; and, most preferably, at least 25vol %, carbon dioxide—is withdrawn. This stream usually contains atleast carbon dioxide, water vapor, nitrogen, and oxygen, as well as theother components mentioned in the Summary section above. Combustionexhaust stream 105 is optionally but typically routed through acondenser 114, where water 110 is knocked out of the stream. Thedehydrated exhaust stream 106 is then typically routed through valve orsplitter 116, and passes as feed stream 108 to a sweep-based membraneseparation step, 111.

The membrane separation unit 111 contains membranes 118 that exhibithigh permeance for carbon dioxide, as well as high selectivity forcarbon dioxide over nitrogen. Any membrane with suitable performanceproperties may be used. Many polymeric materials, especially elastomericmaterials, are very permeable to carbon dioxide. Preferred membranes forseparating carbon dioxide from nitrogen or other inert gases have aselective layer based on a polyether. A number of such membranes areknown to have high carbon dioxide/nitrogen selectivity, such as 30, 40,50, or above. A representative preferred material for the selectivelayer is Pebax®, a polyamide-polyether block copolymer materialdescribed in detail in U.S. Pat. No. 4,963,165. We have found thatmembranes using Pebax® as the selective polymer can maintain aselectivity of 10 or greater under process conditions.

The membrane may take the form of a homogeneous film, an integralasymmetric membrane, a multilayer composite membrane, a membraneincorporating a gel or liquid layer or particulates, or any other formknown in the art. If elastomeric membranes are used, the preferred formis a composite membrane including a microporous support layer formechanical strength and a rubbery coating layer that is responsible forthe separation properties.

The membranes may be manufactured as flat sheets or as fibers and housedin any convenient module form, including spiral-wound modules,plate-and-frame modules, and potted hollow fiber modules. The making ofall these types of membranes and modules is well known in the art. Toprovide countercurrent flow of the sweep gas stream, the modulespreferably take the form of hollow fiber modules, plate-and-framemodules, or spiral-wound modules.

Flat-sheet membranes in spiral-wound modules is the most preferredchoice for the membrane/module configuration. A number of designs thatenable spiral-wound modules to be used in counterflow mode with orwithout sweep on the permeate side have been devised. A representativeexample is described in U.S. Pat. No. 5,034,126, to Dow Chemical.

Membrane step or unit 111 may contain a single membrane module or bankof membrane modules or an array of modules. A single unit or stagecontaining one or a bank of membrane modules is adequate for manyapplications. If the residue stream requires further purification, maybe passed to a second bank of membrane modules for a second processingstep. If the permeate stream requires further concentration, it may bepassed to a second bank of membrane modules for a second-stagetreatment. Such multi-stage or multi-step processes, and variantsthereof, will be familiar to those of skill in the art, who willappreciate that the membrane separation step may be configured in manypossible ways, including single-stage, multi-stage, multi-step, or morecomplicated arrays of two or more units in serial or cascadearrangements.

Although the membrane modules are typically arranged horizontally, avertical configuration may in some cases be preferred to reduce the riskof deposition of particulates on the membrane feed surface.

The separation of components achieved by the membrane unit depends notonly on the selectivity of the membrane for the components to beseparated, but also on the pressure ratio. By pressure ratio, we meanthe ratio of total feed pressure/total permeate pressure. In pressuredriven processes, it can be shown mathematically that the enrichment ofa component (that is, the ratio of component permeate partialpressure/component feed partial pressure) can never be greater than thepressure ratio. This relationship is true, irrespective of how high theselectivity of the membrane may be.

Further, the mathematical relationship between pressure ratio andselectivity predicts that whichever property is numerically smaller willdominate the separation. Thus, if the numerical value of the pressureratio is much higher than the selectivity, then the separationachievable in the process will not be limited by the pressure ratio, butwill depend on the selectivity capability of the membranes. Conversely,if the membrane selectivity is numerically very much higher than thepressure ratio, the pressure ratio will limit the separation. In thiscase, the permeate concentration becomes essentially independent of themembrane selectivity and is determined by the pressure ratio alone.

High pressure ratios can be achieved by compressing the feed gas to ahigh pressure or by using vacuum pumps to create a lowered pressure onthe permeate side, or a combination of both. However, the higher theselectivity, the more costly in capital and energy it becomes to achievea pressure ratio numerically comparable with or greater than theselectivity.

From the above, it can be seen that pressure-driven processes usingmembranes of high selectivity for the components to be separated arelikely to be pressure ratio-limited. For example, a process in which amembrane selectivity of 40, 50, or above is possible (such as is theease for many carbon dioxide/nitrogen separations) will only be able totake advantage of the high selectivity if the pressure ratio is ofcomparable or greater magnitude.

The inventors have overcome this problem and made it possible to utilizemore of the intrinsic selective capability of the membrane by dilutingthe permeate with the sweep gas, stream 101, thereby preventing thepermeate side concentration building up to a limiting level.

This mode of operation can be used with a pressure ratio of 1, that is,with no total pressure difference between the feed and permeate sides,with a pressure ratio less than 1, that is, with a higher total pressureon the permeate side than on the feed side, or with a relatively modestpressure ratio of less than 10 or less than 5, for example.

The driving force for transmembrane permeation is supplied by loweringthe partial pressure of the desired permeant on the permeate side to alevel below its partial pressure on the feed side. The use of the sweepgas stream 101 maintains a low carbon dioxide partial pressure on thepermeate side, thereby providing driving force.

The partial pressure on the permeate side may be controlled by adjustingthe flow rate of the sweep stream to a desired value. In principle, theratio of sweep gas flow to feed gas flow may be any value that providesthe desired results, although the ratio of sweep gas flow:feed gas flowwill seldom be less than 0.1 or greater than 10. High ratios (that is,high sweep flow rates) achieve maximum carbon dioxide removal from thefeed, but a comparatively carbon dioxide dilute permeate stream (thatis, comparatively low carbon dioxide enrichment in the sweep gas exitingthe modules). Low ratios (that is, low sweep flow rates) achieve highconcentrations of carbon dioxide in the permeate, but relatively lowlevels of carbon dioxide removal from the feed.

Use of a too low sweep flow rate may provide insufficient driving forcefor a good separation, and use of an overly high sweep flow rate maylead to pressure drop or other problems on the permeate side, or mayadversely affect the stoichiometry in the reaction vessel, whileachieving only an incremental improvement in separation. Typically andpreferably, the flow rate of the sweep stream should be between about50% and 200% of the flow rate of the membrane feed stream, and mostpreferably between about 80% and 120%. Often a ratio of about 1:1 isconvenient and appropriate.

The total gas pressures on each side of the membrane may be the same ordifferent, and each may be above or below atmospheric pressure. Asmentioned above, if the pressures are about the same, the entire drivingforce for permeation is provided by the sweep mode operation.

In most cases, however, flue gas is available at atmospheric pressure,and the volumes of the streams involved are so large that it is notpreferred to use either significant compression on the feed side orvacuum on the permeate side. However, slight compression, such as fromatmospheric to 2 or 3 bar, can be helpful and can provide part of atotal carbon dioxide capture and recovery process that is relativelyenergy efficient, as shown in the examples below.

Returning again to FIG. 1, flue gas stream 108 flows as a feed streamacross the feed side of the membranes, while a sweep gas of air,oxygen-enriched air, or oxygen stream 101, flows across the permeateside. The sweep stream picks up the preferentially permeating carbondioxide, and the resulting permeate stream 103 is withdrawn from themembrane unit and is combined with stream 115 to form the air or oxygenfeed 104 to the combustor. In the alternative, stream 115 may be omittedand the entirety of the oxygen-containing feed to the combustor may beprovided by the permeate stream 103.

By using the oxygen-containing stream destined for the combustor assweep gas, the membrane separation step is carried out in a veryefficient manner, and without introducing any additional unwantedcomponents into the combustion zone. The process is particularly usefulin applications that are energy-sensitive, as is almost always the casewhen the very large streams from power plants and the like are to beprocessed. The process is also particularly useful in separations thatare pressure-ratio limited.

The residue stream 109 resulting from the membrane sweep step 111 isreduced in carbon dioxide content to less than about 5 vol %, morepreferably, to less than 3 vol %; and, most preferably, to less than 2vol %. The residue stream 109 is typically discharged to the environmentas treated flue gas, but may alternatively be sent on for furthertreatment—membrane or otherwise.

As discussed above, the permeate/sweep stream, 103, returns carbondioxide to the combustor, thereby forming a loop, 119, between thecombustor and the membrane unit in which the carbon dioxideconcentration can build up.

The carbon dioxide concentration in the loop 119 builds to aconsiderably higher level than would be the concentration in the fluegas from a conventional combustion step without the membrane separationstep. Typically, the carbon dioxide concentration in the loop will beenriched several fold, such as three, four, five, or more times,compared with the carbon dioxide concentration that would be found inexhaust gas from a combustion step operated without the loopconfiguration. For example, a natural gas-fired, combined cycle powerplant typically produces a flue gas with about 4-5 vol % carbon dioxide.Using the loop process of the invention, the carbon dioxideconcentration of the flue gas may typically be built up to at leastabout 10, 20, 25, or 30 vol %, or more. Similarly, the exhaust gas froman oil- or coal-fired power plant generally contains about 12-15 vol %carbon dioxide, and can typically be built up to at least about 20, 30,40, 50 or more vol % carbon dioxide in the membrane unit/combustor loop.

In addition to recirculating carbon dioxide, the loop also passes oxygenand nitrogen to the combustion step. Most membrane materials have slightselectivity for oxygen over nitrogen, so a little oxygen from the airsweep stream will tend to counter-permeate to the feed side of themembranes and be lost in the membrane residue stream. In consequence,the concentration of oxygen in the combustor may drop, giving rise tothe possibility of incomplete combustion or other problems. As anindication that the combustion step is being provided with an adequatesupply of oxygen, we prefer the process to be operated so as to providean oxygen concentration of at least about 3 vol % in the exhaust gasstream 106 (based on the composition after water removal).

In some combustion processes, a certain volume of excess air or nitrogenis required to flow unburnt through the combustor to control thecombustion temperature. For example, in combined cycle power plantsusing natural gas as fuel, the ratio of air to fuel in the combustionstep may be about twice the stoichiometric ratio needed for combustionof methane, the surplus air being used to cool the gas sufficiently soas not to damage turbine blades or other plant equipment.

Further, we have discovered that trade-offs exist between the degree ofcarbon dioxide enrichment that can be obtained by the membraneseparation steps, the amount of oxygen lost into the residue stream, andthe membrane area and compression requirements to operate the membraneseparation step. In light of all the above considerations, we prefer tooperate the process to keep the carbon dioxide concentration in theexhaust gas side of the loop at no more than about 60 vol %, and no moretypically than about 50 vol %.

Carbon dioxide is withdrawn from the loop through valve or splitter,116, as partially concentrated, carbon dioxide-enriched product, draw,or bleed stream, 107. Typically, the stream will be withdrawncontinuously, although allowing the carbon dioxide to build up andwithdrawing stream 107 intermittently is within the scope of theinvention.

The flow of gas withdrawn from the loop as stream 107 is usually, butnot necessarily, less than half of the flow of gas in stream 106. Theeffect of varying the comparative ratios of gas withdrawn as stream 107and gas passed on to the membrane unit as stream 108 is illustrated inthe examples. For most applications, it is preferred that between about10 vol % and 50 vol % of the gas flowing in stream 106 be withdrawn asstream 107.

Stream 107 is sent to any containment destination, process, oruse—indicated as box 113 in FIG. 1—that breaks down the carbon dioxideor that stores it in a confined way that reduces, eliminates, or delaysfor a prolonged period of time its emission to the atmosphere.Representative, but non-limiting, uses include algaculture and enhancedoil recovery (EOR).

A particular example of a containment destination in which the carbondioxide is broken down or consumed is one in which photosynthesis canoccur, thereby converting the carbon dioxide to sugars or other organiccompounds and oxygen. Specific examples of such a destination arespecialized greenhouses and facilities used for algaculture.Algaculture—also known as “algae farming”—is a form of aquacultureinvolving the farming of various species of algae. Commercial andindustrial algae cultivation has numerous uses, including production offood ingredients, food, fertilizer, bioplastics, dyes and colorants,chemical feedstock, pharmaceuticals, and algal fuel. Water, carbondioxide, minerals, and light are all important factors in algalcultivation, and different algae have different requirements. The basicreaction in water, however, is carbon dioxide+lightenergy=glucose+oxygen. In subsequent reactions, algae can convert theglucose to lipids (in the the production of biodiesel) or ethanol (inthe production of bioethanol). By converting carbon dioxide to glucoseand oxygen, algaculture can serve as an effective means of pollutioncontrol.

In this case, the carbon dioxide enriched stream may simply be directedinto the enclosed environment where the plants or algae that willutilize it are located. For such applications, a carbon dioxideconcentration in stream 107 up to about 30 vol % is preferred. If thestream is still too dilute when it is withdrawn from the loop, it may besubjected to some additional concentration in a separate membraneseparation step. The stream is preferably fed to the algae farm, ponds,greenhouses, or the like at about atmospheric pressure, so that nothingmore than a blower is required to pass the stream from the gas flow loopto the containment destination. Less preferably, the stream may becompressed, if it needs to be transported by pipeline or trucked to thedestination, for example. By concentrating the carbon dioxide, theefficiency of the algae conversion process is enhanced, and the cost oftransporting the carbon dioxide from the power plant to the algae farmis reduced two- or three-fold.

A particular example of a containment use in which the carbon dioxide isnot converted, but is sequestered for a prolonged period is enhanced oilrecovery (EOR). Enhanced oil recovery is a generic term for techniquesfor increasing the amount of crude oil that can be extracted from an oilfield. Using enhanced oil recovery, 30-60% or more of the reservoir'soriginal oil can be extracted, compared with 20-40% if enhanced oilrecovery is not used. Enhanced oil recovery is typically achieved by gasinjection, chemical injection, microbial injection, or thermal recovery.Gas re-injection is presently the most commonly used approach toenhanced oil recovery. In addition to the beneficial effect of thepressure, this method sometimes aids recovery by reducing the viscosityof the crude oil as the gas mixes with it. Gases used include carbondioxide, natural gas, or nitrogen.

Oil displacement by carbon dioxide injection relies on the phasebehavior of the mixtures of carbon dioxide and crude oil, which arestrongly dependent on reservoir temperature, pressure, and crude oilcomposition. These mechanisms range from oil swelling and viscosityreduction for injection of immiscible fluids (at low pressures) tocompletely miscible displacement in high-pressure applications. In theseapplications, more than half and up to two-thirds of the injected carbondioxide returns with the produced oil and is usually re-injected intothe reservoir to minimize operating costs; the remainder is trapped inthe oil reservoir by various means.

In EOR, the gas injected into the field serves both to help repressurizethe field, and as a solvent to reduce hydrocarbon viscosity and renderthe oil more mobile. In general, any inert gas will suffice forrepressurization, whereas most or all of the solvent capability of thegas is provided by carbon dioxide. For this application, therefore, itis most preferred if the carbon dioxide concentration in stream 107 isat least about 30 vol % and, if possible, to 50 vol % or more. To beuseful for EOR, gas stream 107 will normally require compression beforeit can be used.

Another potential application of the process is carbon dioxidesequestration and enhanced coalbed methane recovery (ECMR).Sequestration of carbon dioxide in unmineable coalbeds is a potentialway to reduce greenhouse gas emissions, while increasing coalbed methaneproduction. Carbon dioxide, together with nitrogen, is compressed andinjected into coalbeds. The carbon dioxide—and to a lesser extent, thenitrogen—displaces adsorbed methane held on internal surfaces of coalparticles. Laboratory and field trials have shown that, for every twovolumes of carbon dioxide adsorbed onto the surface of the coal,approximately one volume of methane is displaced and can be recovered.The displaced methane (with impurities such as carbon dioxide, nitrogen,oxygen, and water) can be collected and processed to form pipelinenatural gas and sent back to the power plant. Optionally, untreated gasor minimally treated gas still containing carbon dioxide can be used asfuel by the power plant. The process has been practiced in the San JuanBasin since 1993, using pipeline carbon dioxide, pure nitrogen, or rawflue gas containing carbon dioxide and nitrogen. Methane productionincreases by as much as ten times with the injection of carbon dioxideand/or nitrogen. Similar processes can be used to recover methane fromcoal mines that are no longer in production.

One of the main cost elements in such processes is the cost oftransporting raw flue gas to coalbed methane injection sites. Byconcentrating the carbon dioxide in the flue gas to 30 to 50 vol %, forexample, a substantial reduction in transportation cost is achieved.

Long-term storage of power plant flue gas in coalbeds is one of the mostattractive carbon dioxide sequestration options. The enhanced productionof natural gas produced when the carbon dioxide is injected provides arevenue source that contributes significantly to covering the cost ofcarbon dioxide separation and transportation to the injection wells.

Yet another category of application is where the carbon dioxide is a rawmaterial for some form of chemical process, for example, the productionof cement and aggregate by aqueous precipitation of calcium andmagnesium. In this process, flue gas containing carbon dioxide isscrubbed with alkaline sea water in a large scrubber. Calcium,magnesium, and other metals in the sea water react with the carbondioxide and are removed as carbonated precipitates. Thisabsorption/contact is a major cost in the process, and the size of thiscost is a function of the carbon dioxide concentration in the flue gassent to the absorber. Increasing the concentration of carbon dioxide inthe flue gas reduces the transportation costs for carbon dioxide to thereactor site and improves the efficiency of the reaction process. Thistechnology is being developed by Calera of Moss Landing, Calif., forexample.

Other uses will be apparent to those of skill in the art based on theteachings herein.

FIG. 2 is a schematic drawing of a flow scheme for an embodiment of theinvention, which is a variant of the flow scheme shown in FIG. 1, inwhich the portion of the exhaust stream being routed to the sweep-basedmembrane separation step is passed through a compressor prior to beingsent to the membrane separation step. Such embodiments are preferred insituations where the energy costs of the compression step can betolerated, such as when energy can be recovered in a turbo-expander, asdiscussed with respect to the series embodiments of the invention above.

From FIG. 2, it may be seen that the process of the inventionincorporates a combustion step, followed by a compression step and asweep-based membrane separation step. A portion of the exhaust streamfrom the combustion process is routed to the sweep-based membraneseparation step, and the permeate portion of gas from the membraneseparation step is routed back to the combustion step, thereby forming agas-flow loop, indicated by number 219 in the figure. A bleed, draw, orproduct stream is withdrawn from the gas flow loop and routed to aprocess that breaks down or stores the carbon dioxide in a confinedmanner, such as an algae farm or enhanced oil recovery process.

To initiate the combustion process, fuel stream 202 and air,oxygen-enriched air, or oxygen stream 204 are introduced into combustionstep or zone 212. Stream 204 is made up of sweep stream 203 andoptionally, additional air or oxygen supply stream 215.

Combustion exhaust stream 205—preferably containing at least 15 vol %;more preferably, at least 20 vol %; and, most preferably, at least 25vol %, carbon dioxide—is withdrawn. Combustion exhaust stream 205 istypically routed through a condenser 214, where water 210 is knocked outof the stream. The dehydrated exhaust stream 206 is then typicallyrouted through a valve or splitter 216, and passes as feed stream 208 toa sweep-based membrane separation step, 211.

In this particular embodiment, the portion 208 of combustion exhauststream 206 that is to be routed to the sweep-based membrane separationstep 211 is routed through a compressor 220, where it is compressed to apressure of up to about 5 bar, such as 2 bar, prior to being sent fortreatment in membrane separation step or unit 211. The membraneseparation unit 211 contains membranes 218 that exhibit high permeancefor carbon dioxide, as well as high selectivity for carbon dioxide overnitrogen, as discussed above with respect to the invention embodimentshown in FIG. 1.

Portion 208 of combustion exhaust stream 206 flows across the feed sideof the membranes; a sweep gas of air, oxygen-enriched air, or oxygenstream 201, flows across the permeate side. The sweep stream picks upthe preferentially permeating carbon dioxide, and the resulting permeatestream 203 is withdrawn from the membrane unit and is combined withstream 215 to form the air or oxygen feed 204 to the combustor. In thealternative, stream 215 may be omitted and the entirety of theoxygen-containing feed to the combustor may be provided by the permeatestream 203.

The residue stream 209 resulting from the membrane sweep step 211 isreduced in carbon dioxide content to less than about 5 vol %, morepreferably, to less than 3 vol %; and, most preferably, to less than 2vol %. The residue stream 209 is typically discharged to the environmentas treated flue gas, but may alternatively be sent on for furthertreatment—membrane or otherwise.

As discussed above, the permeate/sweep stream, 203, returns carbondioxide to the combustor, thereby forming a loop, 219, between thecombustor and the membrane unit in which the carbon dioxideconcentration can build up.

The carbon dioxide concentration in the loop 219 builds to aconsiderably higher level than would be the concentration in the fluegas from a conventional combustion step without the membrane separationstep. Typically, the carbon dioxide concentration in the loop will beenriched several fold, such as three, four, five, or more times,compared with the carbon dioxide concentration that would be found inexhaust gas from a combustion step operated without the loopconfiguration. For example, a natural gas-fired, combined cycle powerplant typically produces a flue gas with about 4-5 vol % carbon dioxide.Using the loop process of the invention, the carbon dioxideconcentration of the flue gas may typically be built up to 10, 20, 25,or 30 vol %. Similarly, the exhaust gas from an oil- or coal-fired powerplant generally contains about 12-15 vol % carbon dioxide, and can bebuilt up to 20, 30, 40, or more vol % carbon dioxide in the membraneunit/combustor loop.

Carbon dioxide is withdrawn from the loop through valve or splitter,216, as partially concentrated, carbon dioxide-enriched product, draw,or bleed stream, 207. Typically, the stream will be withdrawncontinuously, although, as discussed above, allowing the carbon dioxideto build up and withdrawing stream 207 intermittently is within thescope of the invention.

The flow of gas withdrawn from the loop as stream 207 is usually, butnot necessarily, less than half of the flow of gas in stream 206. Theeffect of varying the comparative ratios of gas withdrawn as stream 207and gas passed on to the membrane unit as stream 208 is illustrated inthe examples. For most applications, it is preferred that between about10 vol % and 50 vol % of the gas flowing in stream 206 be withdrawn asstream 207.

Stream 207 is sent to any containment destination, process, oruse—indicated as box 213 in FIG. 2—that breaks down the carbon dioxideor that stores it in a confined way that reduces, eliminates, or delaysfor a prolonged period of time its emission to the atmosphere, asdiscussed above.

The invention is now further described by the following examples, whichare intended to be illustrative of the invention, but are not intendedto limit the scope or underlying principles in any way.

EXAMPLES Example 1 Bases of Calculations for Other Examples

(a) Membrane permeation experiments: The following calculations wperformed using a composite membrane having a polyether-based selectivelayer with the properties shown in Table 1.

TABLE 1 Gas Permeance (gpu)* CO₂/Gas Selectivity Carbon dioxide 1,000  — Nitrogen 30 33 Oxygen 60 17 Hydrogen 100  10 Carbon  1 1,000 Water5,000**  — *Gas permeation unit; 1 gpu = 1 × 10⁻⁶ cm³(STP)/cm² · s ·cmHg **Estimated, not measured

(b) Calculation methodology: All calculations were performed using amodeling program, ChemCad 5.6 (Chem Stations, Inc., Houston, Tex.),containing code for the membrane operation developed by MTR'sengineering group. For the calculations, all compressors and vacuumpumps were assumed to be 75% efficient. in each case, the modelingcalculation was performed to achieve 90% recovery of carbon dioxide fromthe flue gas stream,

(c) “No membrane” example: A computer calculation was performed todetermine the chemical composition of untreated flue gas from a coalcombustion process. The calculation was performed assuming that the fluegas to be treated was from a 500 MW gross power coal-fired power plant.It was assumed that the exhaust gas is filtered to remove fly ash andother particulate matter before passing to the membrane separationsteps.

FIG. 3 is a schematic drawing of a flow scheme for a combustion processthat does not include a sweep-based membrane separation step. Fuelstream 302 and air stream 301 are introduced into combustion step orzone 303. (The combustion step, the fuel for the combustion step, andthe oxygen with which the fuel is combined are as described in theDetailed Description, above.)

Combustion exhaust stream 304 is withdrawn, then routed through acooler/condenser 307, where water 305 is knocked out of the stream. Thechemical composition of the resulting untreated flue gas stream 306 wasthen calculated. The results of this calculation are shown in Table 2.

TABLE 2 Stream Condenser Coal Air Stream Knockout Flue Gas Parameter(302) (301) (305) (306) Total Flow (kg/h) 132,000 2,280,000 62,1602,349,600 Temperature (° C.) 25.0 25.0 40.0 40.0 Pressure (bar) 1.0 1.01.0 1.0 Component (vol %) Coal (carbon + 100.0 0 0 0 hydrogen) Oxygen 021.0 0 3.1 Nitrogen 0 79.0 0 77.8 Carbon Dioxide 0 0 0 11.7 Water 0 0100 7.4

After the water vapor in the stream is condensed, the carbon dioxideconcentration in the combustion exhaust stream is 11.7 vol %. Dischargeof such a stream in its untreated form would release 10,000 tons ofcarbon dioxide into the atmosphere per day. On the other hand, the flowvolume of the stream (1,800,000 m³/h) is so large as to rendertransportation of the stream for treatment or use very difficult.

Examples 2-8 Processes of the Invention: Modeling of Sweep-BasedMembrane Separation Step and Effect on Combustion Step

A set of calculations was performed to model the effect of variousprocess parameters on the performance of the sweep-based membraneseparation step and its effect on the combustion step. The calculationsfor Examples 2 through 8 were performed using the flow scheme shown inFIG. 1 and described in the Detailed Description, above. This flowscheme includes a sweep-based membrane separation step, 111.

To facilitate operation of the calculation software, the base ease airflow provided to the combustor via the membrane permeate side wasassumed to be about 740 m³/h (950 kg/h), compared with the typical airflow to a 500 MW power plant of about 1.8 million m³/h used for thecalculation of Example 1. In other words, the scale of the calculationsfor Examples 2 through 8 was about 1/2,400 of the scale for a typicalcoal-fired power plant. This reduces membrane area proportionately, butdoes not affect the relative flow rates or compositions of the streamsinvolved.

Example 2 Process of the Invention

In this example, the membrane area was assumed to be 400 m², and thecombustion exhaust stream split ratio was set at 1:1 (bleed flow ofpartially concentrate product gas:flow to sweep-based membraneseparation step). The separation was assumed to be performed using amembrane having permeation properties as in Table 1. The results of thiscalculation are shown in Table 3.

TABLE 3 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow 55 46 554 554 950 1100404 (kg/h) Temperature 25 40 40 40 25 33 25 (° C.) Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 1.6 1.6 21.0 17.5 5.2 Nitrogen 0 0 71.8 71.8 79.070.6 91.9 Carbon 0 0 19.2 19.2 0 8.3 2.9 Dioxide Water 0 100 7.4 7.4 03.6 0

Compared with the “no membrane” Example 1, the carbon dioxide content inthe combustion exhaust stream (membrane feed), 108, and the partiallyconcentrated product gas, 107, is increased from 11.7 vol % to 19.2 vol%. The carbon dioxide concentration in the membrane residue stream isreduced to 2.9 vol %, and venting of this stream to the atmosphere wouldrelease about 1,000 tons of carbon dioxide per day from a 500 MW powerplant. Comparing this Example with Example 1, it can be seen that theprocess is effective in capturing 90% of the carbon dioxide emitted fromthe combustion section of the power plant.

As can also be seen, however, use of the incoming air as the permeatesweep stream reduces the oxygen content in the air to the combustor fromthe normal 21 vol % to 17.5 vol %. As a result, the oxygen content ofthe combustion exhaust stream, 108, is reduced to 1.6 vol %. The lowexhaust oxygen concentration indicates that the combustion process maybe compromised under these conditions.

Example 3 Process of the Invention with Increased Air Flow

In this set of calculations, the air flow to the process via thepermeate sweep stream was increased incrementally, until the calculationshowed an oxygen content of 3 vol % in the combustor exhaust stream.This required the intake flow rate of air to be increased from 950 kWhto 1,035 kg/h. All other operating parameters, including split ratio andmembrane area, were the same as in Example 2. The results of thecalculation are shown in Table 4.

TABLE 4 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow 55 44 596 596 1035 1182450 (kg/h) Temperature 25 40 40 40 25 33 25 (° C.) Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 3.2 3.2 21.0 17.8 6.6 Nitrogen 0 0 72.0 72.0 79.071.1 90.3 Carbon Dioxide 0 0 17.4 17.4 0 7.4 3.1 Water 0 100 7.4 7.4 03.7 0

Although the oxygen content of the combustion exhaust stream 108 isincreased to 3.2 vol %, the carbon dioxide content of the combustionexhaust stream, 108, and, hence, the partially concentrated product gas,107, was lower than m Example 2, at 17.4 vol % and the carbon dioxidecontent of the treated flue gas, 109, was a little higher, at 3.1 vol %.

Example 4 Process of the Invention with Split Ratio 1:2

In this set of calculations, the split ratio was changed to 1:2, thatis, two volumes of exhaust gas were assumed to be sent to the membraneseparation step for every volume of exhaust gas withdrawn from the loopas partially concentrated product gas. The membrane area as againassumed to be 400 m², and the air flow rate in stream 101 was assumed tobe the base calculation value of 950 kg/h. The results of thecalculation are shown in Table 5.

TABLE 5 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow 55 54 384 768 950 1151567 (kg/h) Temperature 25 40 40 40 25 35 25 (° C.) Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100.0 0 0 0 0 00 hydrogen) Oxygen 0 0 1.6 1.6 21.0 16.8 4.3 Nitrogen 0 0 69.7 69.7 79.067.8 89.2 Carbon Dioxide 0 0 21.3 21.3 0 10.6 6.4 Water 0 100 7.4 7.4 04.8 0.1

Increasing the relative volume of exhaust gas being treated in themembrane separation step results in a substantial increase in theconcentration of carbon dioxide in the partially concentrated productgas. The exhaust gas has almost double the concentration of carbondioxide compared with Example 1. However, the oxygen content of thecombustion exhaust stream, 108, has dropped to an undesirably, low value(1.6 vol %) and the carbon dioxide content of the treated flue gas isrelatively high (6.5 vol %).

Example 5 Process of the Invention with Increased Membrane Area

To compensate for the adverse aspects of decreasing the split ratio asin Example 4, the membrane area was assumed to be doubled, to 800 m². Asin Example 3, a set of calculations was performed, increasing the airintake incrementally to the process to bring the oxygen concentration ofthe flue gas stream from the combustor back to 3 vol %. This requiredthe intake flow rate of air to be increased from 950 kg/h to 1,090 kg/h.All other operating parameters, including split ratio, were the same asin Example 4. The results of the calculation are shown in Table 6.

TABLE 6 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow (kg/h) 55 51 456 912 10901364 638.1 Temperature (° C.) 25 40 40 40 25 35 25 Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 2.9 2.9 21.0 16.0 7.3 Nitrogen 0 0 67.8 67.8 79.066.5 89.6 Carbon Dioxide 0 0 21.8 21.8 0 12.7 3.1 Water 0 100 7.4 7.4 04.8 0

Compared to Example 4, the carbon dioxide content of the partiallyconcentrated product gas, 107, remained high (21.8 vol %). The oxygencontent of the combustion exhaust stream, 108, was raised to almost 3vol %, and the carbon dioxide content of the treated flue gas, 109, wasreduced to a much lower level of 3.1 vol %. The process achieves about82% recovery of carbon dioxide. The cost of transporting the carbondioxide exhaust gas to some off-site use is cut almost in half comparedto the cost of transporting untreated flue gas.

Example 6 Process of the Invention with Split Ratio 1:4

In this example, the split ratio was changed to 1:4; that is, fourvolumes of exhaust gas were assumed to be sent to the membraneseparation step for every volume of exhaust gas sent to the algae farmor enhanced oil recovery process. The membrane area was again assumed tobe 400 m², and the air flow rate in stream 101 was assumed to be thebase case calculation value of 950 kg/h. The results of the calculationare shown in Table 7.

TABLE 7 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow 55 59 236 942 950 1182710 (kg/h) Temperature (° C.) 25 40 40 40 25 37 25 Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 1.6 1.6 21.0 16.4 3.8 Nitrogen 0 0 68.5 68.5 79.066.3 86.8 Carbon Dioxide 0 0 22.5 22.5 0 11.8 9.1 Water 0 100 7.4 7.4 05.5 0.3

Increasing the relative volume of exhaust gas being treated in themembrane separation step results in a further increase in the carbondioxide content of the partially concentrated product gas, 107, to 22.5vol %; however, the oxygen content of the combustion exhaust stream,108, is low (1.6 vol %) and the carbon dioxide content of the treatedflue gas, 109, is high (9.1 vol %).

Example 7 Process of the Invention

As with Example 5, the membrane area and air intake were assumed to beincreased to balance the less desirable effects of decreasing the splitratio. In this example the membrane area was assumed to be increased to1,600 m², and the air flow, 101, was assumed to be increased to 1,200kg/h. Other parameters are as in Example 6, including a split ratio of1:4. The results of the calculation are shown in Table 8

TABLE 8 Stream Partially Condenser Concentrated Membrane Air Gas to CoalKnockout Product Gas Feed Sweep Combustor Retentate Parameter (102)(110) (107) (108) (101) (103) (109) Total Flow 55 56 340 1361 1200 1702859 (kg/h) Temperature (° C.) 25 40 40 40 25 37 25 Pressure (bar) 1.01.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 2.9 2.9 21.0 13.6 8.5 Nitrogen 0 0 62.0 62.0 79.060.8 88.1 Carbon Dioxide 0 0 27.7 27.7 0 19.9 3.3 Water 0 100 7.4 7.4 05.7 0.1

The combination of the lowered split ratio, increased membrane area, andincreased air flow rate resulted in an increase of the carbon dioxidecontent of the partially concentrated product gas, 107, to 27.7 vol %.The oxygen content of the combustion exhaust stream, 108, was about 3vol %, and the carbon dioxide content of the treated flue gas waslowered to 3.3 vol %. The process achieves about 75% carbon dioxiderecovery.

Example 8 Process of the Invention with Addition of Oxygen

The process of the invention was assumed to be carried out as in Example7. The only difference was that the an intake via the permeate sweepline was assumed to be at the base case flow rate value of 950 kg/h, andan additional 50 kg/h of pure oxygen was assumed to be introduceddirectly into the combustor as stream 115. The results of thecalculation are shown in Table 9.

TABLE 9 Stream Partially Conc'd Condenser Product Membrane Air Gas toCoal O₂ Knockout Gas Feed Sweep Combustor Retentate Parameter (102)(101) (110) (107) (108) (101) (103) (109) Total Flow 55 50 58 306 1225950 1485 690.4 (kg/h) Temperature (° C.) 25 25 40 40 40 25 37 25Pressure (bar) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Component (vol %) Coal(carbon + 100 0 0 0 0 0 0 0 hydrogen) Oxygen 0 100 0 3.0 3.0 21.0 12.49.7 Nitrogen 0 0 0 56.1 56.1 79.0 56.4 87.0 Carbon Dioxide 0 0 0 33.533.5 0 25.2 3.3 Water 0 0 100 7.4 7.4 0 6.0 0

With addition of make-up oxygen, the carbon dioxide content of thepartially concentrated gas product, 107, is increased to 33.5 vol %. Theoxygen content of the combustion exhaust stream, 108, is 3.0 vol %, andthe carbon dioxide content of the treated flue gas, 109, is 3.3 vol %.The process achieves about 80% carbon dioxide recovery.

Example 9-10 Process of the Invention with Compression of Membrane FeedStream

The calculations for Examples 9 and 10 were performed using the flowscheme shown in FIG. 2 and described in the Detailed Description, above.The flow scheme shown in FIG. 2 is a variant of the flow scheme shown inFIG. 1, in which the portion of the exhaust stream, 208, being routed tothe sweep-based membrane separation step, 211, is passed through acompressor, 219, prior to being sent to the membrane separation step,211.

Example 9 Process of the Invention with Split Ratio 1:4

A calculation was performed using the results of Example 7 as basis, andagain assuming a split ratio of 1:4. Iterative calculations showed thatuse of feed compression to 2 bar enables the membrane area to be reducedto 500 m², and the air flow, 201, to be reduced to 1050 kg/h. Theresults of the calculation are shown in Table 10.

TABLE 10 Stream Partially Condenser Concentrated Membrane Air Gas toCoal Knockout Gas Product Feed Sweep Combustor Retentate Parameter (202)(210) (207) (208) (201) (203) (209) Total Flow 55 63 327 1309 1050 1644715 (kg/h) Temperature (° C.) 25 30 30 45 25 40 25 Pressure (bar) 1.01.0 1.0 2.0 1.0 1.0 2.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 2.9 2.9 21.0 14.3 5.9 Nitrogen 0 0 60.7 60.7 79.058.9 91.2 Carbon Dioxide 0 0 32.1 32.1 0 23.5 2.9 Water 0 100 4.3 4.3 03.3 0

The carbon dioxide content of the partially concentrated gas product,207, is 32.1 vol %. the oxygen content of the combustion exhaust stream,208, is just under 3 vol, and the carbon dioxide content of the treatedflue gas, 209, is 2.9 vol %. Recovery of carbon dioxide is about 85%.

Example 10 Process of the Invention with Split Ratio 1:10

This Example illustrates the combined effect of slight compression and alow split ratio of 1:10. The calculation was performed assuming amembrane area of 600 m², and an intake air flow rate in stream 201 of1,150 kg/h. The assumption for membrane area and air intake flow rateare higher than for Example 9, to balance the lower split ratio. Theresults of the calculation are shown in Table 11.

TABLE 11 Stream Partially Condenser Concentrated Membrane Air Gas toCoal Knockout Gas Product Feed Sweep Combustor Retentate Parameter (202)(210) (207) (208) (201) (203) (209) Total Flow 55 65 232 2085 1150 2327909 (kg/h) Temperature (° C.) 25 30 30 45 25 43 25 Pressure (bar) 1.01.0 1.0 2.0 1.0 1.0 2.0 Component (vol %) Coal (carbon + 100 0 0 0 0 0 0hydrogen) Oxygen 0 0 3.2 3.2 21.0 11.7 7.2 Nitrogen 0 0 50.1 50.1 79.048.9 89.3 Carbon Dioxide 0 0 42.4 42.4 0 35.7 3.5 Water 0 100 4.3 4.3 03.7 0

The carbon dioxide content of the partially concentrated gas product,207, is increased to 42.4 vol %. The oxygen content of the combustionexhaust stream, 208, is 3.2 vol %, and the carbon dioxide content of thetreated flue gas, 209, is 3.5 vol %. The recovery of carbon dioxide inthis case drops to about 65%.

We claim:
 1. A membrane process for treating flue gas, comprising: (a)performing a combustion step by combusting a mixture of a fuel and anoxygen-enriched air, or oxygen, thereby creating an exhaust streamcomprising carbon dioxide and nitrogen; (b) splitting the exhaust streaminto a first portion and a second portion without adjusting thecomposition of either portion; (c) providing a membrane having a feedside and a permeate side, and being selectively permeable to carbondioxide over nitrogen and to carbon dioxide over oxygen; (d) performinga membrane separation step, comprising, (i) passing a first portion ofthe exhaust stream across the feed side, (ii) passing air,oxygen-enriched air, or oxygen as a sweep stream across the permeateside, (iii) withdrawing from the feed side a carbon dioxide-depletedvent stream, and (iv) withdrawing from the permeate side a permeatestream comprising oxygen and carbon dioxide; (e) passing at least aportion of the permeate stream to step (a) as at least part of the air,oxygen-enriched air, or oxygen used in step (a), thereby forming agas-flow loop between the combustion step and the membrane separationstep; (f) withdrawing a second portion of the exhaust stream from apoint in the gas flow loop downstream of the combustion step andupstream of the membrane separation step; and (g) using, storing, orotherwise disposing of the second portion in a confined manner.
 2. Theprocess of claim 1, wherein the second portion is transported to aconfining operation, where it is used, stored, or otherwise disposed ofin a confined manner.
 3. The process of claim 1, wherein the secondportion is used for enhanced oil recovery.
 4. The process of claim 1,wherein the second portion is used for enhanced coalbed or coal minemethane recovery.
 5. The process of claim 1, wherein the second portionis used in the production of carbonates.
 6. The process of claim 1wherein the second portion is injected into subsurface water.
 7. Theprocess of claim 1, wherein the second portion comprises at least 20 vol% CO₂.
 8. The process of claim 7, wherein the second portion comprisesat least 30 vol % CO₂.
 9. The process of claim 8, wherein the secondportion comprises at least 40 vol % CO₂.
 10. The process of claim 7,wherein the second portion comprises less than 60 vol % CO₂.
 11. Theprocess of claim 1, wherein the second portion comprises at least 3 vol% oxygen.
 12. The process of claim 1, wherein between about 10 vol % and50 vol % of the exhaust stream is withdrawn from the gas flow loop asthe second portion.
 13. The process of claim 1, wherein the firstportion is compressed o a pressure of up to about 5 bar before beingpassed across the feed side of the membrane.
 14. The process of claim 1,wherein the membrane exhibits a carbon dioxide permeance of at least 500gpu under process operating conditions.
 15. The process of claim 1,wherein the membrane exhibits a selectivity in favor of carbon dioxideover nitrogen of at least 10 under process operating conditions.
 16. Theprocess of claim 1, wherein the vent stream comprises 3 vol % carbondioxide or less.
 17. The process of clam 16, wherein the vent streamcomprises 2 vol % carbon dioxide or less.
 18. The process of claim 1,wherein the membrane comprises two or more membranes, and the two ormore membranes are arranged in one or more modules, and wherein the oneor more modules are arranged in a vertical configuration.
 19. A membraneprocess for treating flue gas, comprising: (a) performing a combustionstep by combusting a mixture of a fuel and air, oxygen enriched air, oroxygen, thereby creating an exhaust stream comprising carbon dioxide andnitrogen; (b) splitting the exhaust stream into a first portion and asecond portion without adjusting the composition of either portion; (c)providing a membrane having a feed side and a permeate side, and beingselectively permeable to carbon dioxide over nitrogen and to carbondioxide over oxygen; (d) performing a membrane separation step,comprising, (i) passing a first portion of the exhaust stream across thefeed side, (ii) passing air, oxygen-enriched air, or oxygen as a sweepstream across the permeate side, (iii) withdrawing from the feed side acarbon dioxide-depleted vent stream, and (iv) withdrawing from thepermeate side a permeate stream comprising oxygen and carbon dioxide;(e) passing at least a portion of the permeate stream to step (a) as atleast part of the air, oxygen-enriched an or oxygen used in step (a),thereby forming a gas-flow loop between the combustion step and themembrane separation step; (f) withdrawing a second portion of theexhaust stream from a point in the gas flow loop downstream of thecombustion step and upstream of the membrane separation step; and (g)using the second portion for enhanced oil recovery.
 20. The process ofclaim 19, wherein the second portion comprises at least 30 vol % CO₂.21. The process of claim 20, wherein the second portion comprises atleast 40 vol % CO₂.
 22. The process of claim 21, wherein the secondportion comprises at least 50 vol % CO₂.